1. Field of the Invention
The present invention relates to a color conversion definition creating apparatus that creates a color conversion definition which defines a color conversion relationship between a first color expression area which expresses a range of colors that can be expressed by a first device which mediates between an image and image data and a second color expression area which expresses a range of colors that can be expressed by a second device which mediates between an image and image data, a color conversion definition creating program storage medium that stores a color conversion definition creating program for causing a computer to operate as the color conversion definition creating apparatus, and a color conversion definition creating method to create a color conversion definition.
2. Description of the Related Art
Various types of input device which obtains image data by inputting an image, for example, a color scanner which reads a recorded image and obtains image data and DSC (digital still camera) which obtains image data by forming an image of an object on a solid image-pickup device and reads the image are known. In these input devices, image data is expressed by data in a predetermined range of, for example, 0 to 255 for each of 3 colors of R (red), G (green), B (blue), but there are naturally limitations on colors that can be expressed with numerical values within the respective predetermined ranges of 3 colors of R, G and B and even if colors of an original image have an extremely abundant expressions, when the colors are converted to image data using an input device, the colors of the image expressed by the image data are limited to colors within certain color expression areas within color spaces of R, G, B.
On the other hand, various types of output device which outputs an image based on image data such as a photograph printer which records an image onto photographic paper by exposing the surface of the photographic paper to laser light and developing the photographic paper, a printer which records an image on a sheet using a system such as an electrophotographic system or an ink jet system, a printing machine which turns a rotary press and creates many prints, an image display device such as a CRT display or a plasma display which displays an image on a display screen based on image data are known, but even for these output devices, there are also color expression areas corresponding to each output device as in the case of the input devices. That is, an output device can express various colors based on image data which expresses 3 colors of R, G, B and image data which expresses 4 colors of C (cyan), M (magenta), Y (yellow), K (black), but the colors which can be expressed are limited to certain color expression areas (for example, color expression area which is expressed by numerical values within a range of 0 to 255 for each of R, G, B) in the output device color space (for example, RGB color space, CMYK color space). A color expression area in such an input device and output device is called “color gamut.”
On the other hand, in recent years, there are an increasing number of cases where image data is diverted between various input devices and output devices and a method of keeping colors of an image equal when diverting image data is also known. For example, a typical method of defining a correlation between coordinates of a device color space which is dependent on the device and coordinates of an independent color space (Device Independent Data space: e.g., L*a*b* color space) which is not dependent on the device by a correspondence table called a “profile”, preparing the table for each type of input device and output device, creating a combined profile which combines profiles of the respective devices for which the image data is diverted and thereby-converting the image data is known.
However, as described above, the color expression area varies from one device to another, and if image data is diverted using the method of keeping image colors equal, some color expressions may be lost in parts where color expression areas do not match. When such a loss is large, the image expressed by the inverted image data becomes an unnatural image.
On the other hand, it is an experimentally known fact that regardless of different color expression areas, an originally identical image can be expressed as images natural to human eyes by various devices respectively. The natural images expressed by these various kinds of devices are expressed in slightly different colors in accordance with the difference among color expression areas of the devices, but since the level of adaptability of human eyes is high, images expressed by any devices give natural impression.
Therefore, there is a demand for a color conversion which coverts colors of an image that can keep an impression of the image natural when inverting image data. Such a color conversion is preferably such that each color in a color expression area (color gamut) of a certain device can be associated with each color in the color expression area (color gamut) of another device in just proportion and this color conversion is called “gamut mapping.”
As described above, the color expression area varies from one device to another and even if image data are numerically the same, their colors expressed by different devices differ from one another. Thus, when converting image data obtained using an input device to image data which is suitable for image output using an output device, a conversion of the image data using a series of procedures as shown below is adopted. First, the image data obtained using the input device is converted to image data in a mapping area on the input side in which the color expression area of the input device is mapped onto a common color space such as an L*a*b* color space. Next, through a gamut mapping between the mapping area on this input side and the mapping area on the output side in which the color expression area of the output device is mapped onto the common color space, the image data on the mapping area on the input side is converted to the image data in the mapping area on the output side. The image data in the mapping area on the output side is converted to image data in the color space which is dependent on the output device.
A color conversion relationship between the color expression area of the input device in which a gamut mapping in such a common color space is incorporated and the color expression area of the output device is defined beforehand in the form of, for example, LUT (lookup table). Then, when a document image is read by the input device, the document image is reproduced and output by the output device, the image data obtained by the input device is subjected to a color conversion using the LUT and an image based on the image data after the color conversion is output by the output device. Here, the LUT incorporating a gamut mapping or the like which defines the color conversion relationship between the color expression area of the input device and the color expression area of the output device is called a “color conversion definition.”
The gamut mapping incorporated in the color conversion relationship defined by such a color conversion definition is roughly divided into: a stage of adaptive conversion in which the color expression area of the input device is converted to an intermediate area which is similar to the color expression area of the output device to a certain degree, by typically approximating a gray area in the color expression area of the input device to the gray area in the color expression area of the output device; and a stage of mapping processing which assigns each color in the intermediate area obtained through this adaptive conversion to the color expression area of the output device based on a predetermined rule. Here, an appreciator's impression about colors of the image finally output from the output device largely depends on the shape of the intermediate area. Furthermore, the shape of the intermediate area is determined in many cases by what adaptive conversion the intermediate area is created, and therefore various gamut mappings with adaptations added to this adaptive conversion are conventionally proposed.
For example, there is a proposal of a gamut mapping which performs a so-called “Vonkries” conversion at the time of an adaptive conversion (for example, see Japanese Patent Laid-Open No. 2001-103329). According to an adaptive conversion which performs this Vonkries conversion, a gray area in the color expression area of the input device approximates to a gray area in the color expression area of the output device by approximating white points in the color expression area of the input device to white points in the color expression area of the output device. Furthermore, when the gray area in the color expression area of the input device approximates to the gray area in the color expression area of the output device through this adaptive conversion, the areas other than the gray area also move according to the movement of the gray area. In addition to the processing about white points in the color expression area of the input device, this Japanese Patent Laid-Open No. 2001-103329 also describes a gamut mapping which performs processing of approximating black points in this color expression area to black points in the color expression area of the output device and improving the degree of approximation of the gray area during an adaptive conversion.
Furthermore, for example, Japanese Patent Laid-Open No. 2002-16816 and Japanese Patent Laid-Open No. 2004-200938 propose a gamut mapping which performs an adaptive conversion which approximates the gray area in the color expression area of the input device to the gray area in the color expression area of the output device by matching brightness of each of the white points and black points in the color expression area of the input device to brightness of each of the white points and black points in the color expression area of the output device while maintaining chroma of each color in the color expression area of the input device.
Furthermore, there is also a proposal of a gamut mapping (for example, see Japanese Patent Laid-Open No. 2003-18416 and Japanese Patent Laid-Open No. 2003-298865) which determines the degree of the adaptation which indicates to what degree the gray area in the color expression area of the input device is approximated to the gray area in the color expression area of the output device according to a predetermined rule, for example, during an adaptive conversion and performs an adaptive conversion which approximates the gray area in the color expression area of the input device to the gray area in the color expression area of the output device based on the determined degree of adaptation.
When a document which is input to the input device is reproduced by the output device, the image which is output by the output device is preferably such an image that maintains the chroma of the document to a certain degree and restrains collapse of gradation on the highlight side and the shadow side or the like. However, when, for example, an image is output by the output device whose brightness range is narrower than the brightness range of the input device, an adaptive conversion such as compressing the brightness range of the input device is performed by matching each of the white points and black points in the color expression area of the input device to each of the white points and black points in the color expression area of the output device in order to restrain the collapse of gradation or the like on the highlight side and the shadow side. However, in the Vonkries conversion or the like, when the brightness range is compressed, the chroma range is also compressed accordingly and depending on the degree of compression of the brightness range, the image which is output by the output device has greater detail on the shadow side, whereas it may become an unnatural, less colorful image with total chroma reduced.
Furthermore, in the gamut mapping described in Japanese Patent Laid-Open No. 2002-16816 and Japanese Patent Laid-Open No. 2004-200938, the chroma of colors in the color expression area of the input device is maintained during an adaptive conversion in order to restrain such a reduction of chroma. According to such a gamut mapping, even if the brightness range of the input device is compressed during the adaptive conversion, the reduction of the chroma in the image which is output by the output device is restrained compared to the adaptive conversion using the Vonkries conversion. However, when the brightness range of the color expression area of the output device is extremely narrower than the brightness range of the color expression area of the input device, the color expression area of the input device is greatly compressed only in the brightness direction during the adaptive conversion, the intermediate area obtained through the adaptive conversion becomes an area where the chroma of colors is unnaturally emphasized. As a result, the image output by the output device becomes an image with unnaturally emphasized chroma.
As described above, various conventional gamut mappings have their advantages and disadvantages and it is the present situation that no such technique that an image with natural colors is output by an output device with a high degree of accuracy is proposed yet.